Rename CallConv::Native to CallConv::SystemV. (#291)

To keep cross-compiling straightforward, Cretonne shouldn't have any
behavior that depends on the host. This renames the "Native" calling
convention to "SystemV", which has a defined meaning for each target,
so that it's clear that the calling convention doesn't change
depending on what host Cretonne is running on.

cranelift/filetests/regalloc/coalesce.cton

@@ -109,7 +109,7 @@ ebb1(v10: i32):
     return v11
 }
 
-function %gvn_unremovable_phi(i32) native {
+function %gvn_unremovable_phi(i32) system_v {
 ebb0(v0: i32):
     v2 = iconst.i32 0
     jump ebb2(v2, v0)